384 ^WE ANIMAL KINGDOM 



fibrous sheath encasing many vacuolated cells, whose turgidity makes it 

 firm yet flexible. It is generally assumed that the notochord provides 

 support for the body, but it can be argued that the small, marine 

 chordates, which first acquired a notochord, did not need this extra 

 support. A more plausible suggestion is that it prevents the body from 

 shortening in the manner of an earthworm when the longitudinal 

 muscle fibers in the body wall contract. Since telescoping is prevented, 

 the contraction of muscle fibers first on one side and then on the other 

 causes the animal to bend from side to side and move through the 

 water with fishlike, lateral undulations. Undulatory movements are 

 possible without a rod of this type— certain marine worms, for example, 

 swim in this fashion— but the notochord may increase the efficiency and 

 precision of this type of locomotion. 



Secondly, a longitudinal nerve cord lies dorsal to the notochord. 

 It differs from the ventral nerve cord of certain nonchordates, both in 

 position and in structure, for it is a single rather than a double cord, 

 and is tubular rather than solid. 



Finally, chordates differ from most nonchordates in having pha- 

 ryngeal pouches that extend laterally from the anterior part of the 

 digestive tract toward the sides of the body, often breaking through as 

 gill slits. All chordates have gill slits, or at least pharyngeal gill 

 pouches, at some stage of their life cycle. Certain hemichordates (p. 360) 

 also have gill slits. This arrangement appears to have served originally as 

 a means of letting the water taken into the mouth escape from the 

 digestive tract, thereby concentrating the small food particles that were 

 in the water. The lower chordates and the larvae of the most primitive 

 vertebrates are food-sifters, or filter-feeders, and live upon minute or- 

 ganic matter gathered in this way. 



In addition to these diagnostic features, chordates share many 

 other characters with certain of the more advanced, nonchordate groups. 

 They are bilaterally symmetrical; they are triploblastic; their general 

 plan of body organization is a tube within a tube, for in most chordates 

 a coelom separates the digestive tract from the body wall; the gut tube 

 is complete, i.e., there is a separate mouth and anus. Diffusion is ade- 

 quate for gas exchange and excretion in the simpler chordates, but 

 special respiratory and excretory organs are present in the vertebrates. 

 The vertebrates are active animals, with a high degree of cephalization 

 (accumulation of nerves and sense organs in the head) and segmental 

 muscular and related systems. 



175. Subphylum Urochordata 



The first chordate subphylum, the Urochordata, includes the marine 

 tunicates and their allies. Most urochordates belong to the class Ascidi- 

 acea, and are sessile organisms that are frequently seen attached to sub- 

 merged rocks and wharf pilings, or are found partially buried in sand 

 and mud in coastal waters. They may be either solitary or colonial 

 (Fig. 20.1). Molgula is a familiar example of the former type occurring 



